[tex] \frac{1}{7}+[ \frac{3}{7}-( \frac{5}{28}- \frac{5}{84})]+ \frac{1}{21}= \frac{3}{21}+ \frac{1}{21} + [ \frac{3}{7}-( \frac{15}{84}- \frac{5}{84})] = \frac{4}{21}+( \frac{3}{7}- \frac{10}{84}) = \frac{16}{84}+ \frac{36}{84}- \frac{10}{84}= \frac{42}{84}= \frac{1}{2} [/tex]
